The technology for controlling the transmission of vibrational energy between a disturbance source such as an engine or powerplant and a support structure such as a pylon or airframe of an aircraft has been continuously evolving. Initially, engines were directly or "hard" mounted to the structure. With such a mount, the base structure experiences all vibrations of the engine. In the first attempts at isolation, solid blocks of rubber were used to isolate an aircraft engine from its support pylon for fixed wing applications or from the air frame for rotary wing utilization. An early example of these elastomeric mounts is taught in U.S. Pat. No. 3,288,404 issued to Schmidt. While softner elastomer sections were preferred for their isolation characteristics, the stiffer elastomer sections were preferred for their durability and stability in maintaining the position of the engine.
Second generation mounts included fluids in elastomeric chambers, the fluids being relied upon to add damping, i.e., to dissipate energy through absorbing the vibrational impact by resisting motion. An example of such a fluid mount is shown in U.S. Pat. No. 3,415,470 issued to Woodford et al. This patent discloses the use of a viscous fluid to provide damping for low frequency, high amplitude vibrations.
In U.S. Pat. No. 4,236,607, Halwes et al. recognized that fluid in mounts had potential utility beyond its use as damping material. The fluid could be used as a mass to alter vibrational characteristics of the system by employing the inertia created by the fluid mass. An inner housing functions as a piston to throttle fluid through a small passage, thus amplifying the induced inertia forces of the fluid movement. One of the benefits of a fluid inertia mount is the ability to create a notch in the dynamic stiffness curve which can be designed or tuned to match a common operating frequency of the source or the system, thereby reducing the force transmitted at that frequency.
While offering some advantages over the conventional fluidless elastomeric mounts, fluid damping and inertia type mounts are both passive, that is, they cannot be adjusted to accommodate variations in the system. Development continued along two separate paths at this point: active mounts and adaptive mounts. The latter will be discussed first.
An adaptive mount behaves differently depending upon conditions. The term "adaptive" embraces a number of very different concepts. Generally, an adaptive mount changes one of the parameters of the mount in order to change its behavior. For example, in a mount having alternative fluid flow paths, the behavioral characteristics of the flow paths can differ significantly. One group of mounts for automative application employ a decoupler mechanism. At low amplitude, medium to high frequency applications, a major portion of the fluid will be pumped around the decoupler, or in some cases, oscillate the decoupler through a short stroke within a large diameter orifice. When a high amplitude, low frequency disturbance occurs, such as a shock load, the decoupler blocks off flow causing the fluid to transit a lengthy, small diameter inertia track where, in essence, the fluid is throttled through the inertia track and undergoes significant damping. Such a mount is shown in U.S. Pat. No. 4,709,907 issued to Thorn.
Another way to change the characteristics of the mount is to change the dimensions of the inertia track. This can include changing the length, the effective cross section, or both. One such mount is disclosed in U.S. Pat. No. 4,969,632 issued to Hodgson et al.
Yet a third way to change mount characteristics is to vary the yeild stress of the fluid. To effect a significant variation, a fluid that is susceptible to such yield stress changes, such as an electrorheological (ER) fluid, for example, should be used. Such a mount is shown in U.S. Pat. No. 5,029,823 issued to Hodgson et al.
As previously mentioned, parallel development was occurring in active mounting systems. Examples of such active systems are found in U.S. Pat. Nos. 4,033,541 to Malueg for use in controlling a payload platform, 3,477,665 to Legrand for use on helicopters, and 4,869,474 to Best et al. for use in automotive mounts. The operation of an active mount involves the sensing of some output parameter of the system and, using an extrinsic power source, inputting energy to counter the effect of the vibrational energy of the source. While these active systems may be suitable for some applications, those which have been developed to date are complex and as they may be used for fixed and rotary wing applications, require a very large power source to directly counter the vibrational energy of the large, high-powered engines.
Recently, attempts have been made to apply noise cancellation technology to structural vibration problems. Some attempts involve evaluating the vibrational energy as it reemerges as sound (in the cabin of an aircraft, for example) and producing a second sound wave 180.degree. out of phase to cancel the noise transmission. Other attempts impart the second vibrational wave which is 180.degree. out of phase directly into the structure to cancel the vibration. See, for example, U.S. Pat. No. 4,562,589 issued to Warnaka et al. The difficulties associated with either approach should be apparent since, when the cancellation signal has a source space from the disturbance source, what is 180.degree. out of phase at one spatial location, will have a different phase relationship (and, therefore, not fully cancel) at another location. Further, by inputting additional energy into the structure, this solution aggravates, rather than alleviates, problems associated with material stresses, structural fatigue, etc. While not all applications will permit the use of an isolation solution due to the primary path of vibration transmission, as in the case of Warnaka et al., where the primary path is through an "energy bottleneck", an isolation mount becomes a perferable solution.
Before describing the features of the present invention, another development should be mentioned. The fluid mounts mentioned above are generally of the single pumper variety, that is, they pump fluid from a first chamber to a second chamber and rely on pressure equalization (i.e., suction) to return the fluid to the first chamber. When pulling a column of fluid, particularly at the frequencies and amplitudes associated with an aerospace mount, there is considerable risk of cavitation, separation of the fluid due to a pressure lower than the vapor pressure of the fluid which is being urged to move. In U.S. Pat. No. 4,811,919 to Jones, a double pumper mount is disclosed in which fluid is pumped to and from a second chamber from and to a first chamber by a two-sided, double-acting piston. When combined with the air-over hydraulic volume compensator, the double pumper mount virtually eliminates the risk of cavitation for most operating conditions since the fluid is under compression, never in tension, as it is pushed. A further distinction of the double pumper mount is that the volume stiffnesses of the two fluid chambers are substantially similar (i.e., as equal as possible) whereas the volume stiffnesses of the chambers of a single pumper mount are significantly (generally, several orders of magnitude) different.
The present invention is an improved active double pumper mount. An actuator is provided in parallel or in series with the passive inertia track to vary the rate of fluid oscillation in the inertia track to control the amount of vibrational energy transmitted between the source of vibrational energy and its base in a desired manner. The passive inertia track will have a characteristic notch at a frequency the isolation system is being designed to control. In the case of an isolator, flow can be altered by the actuator to drive the dynamic stiffness at the notch to zero, that is, to counteract the parasitic damping forces created by the elastomer and the resistance to flow in the fluid passageways. This sytem takes advantage of the amplification made available by the doulbe-acting piston and the fact that the passive notch lessens the dynamic stiffness at this critical frequency. Therefore, a key feature of this system is that it is more efficient than earlier active mounts, requiring less power input and, hence, a smaller power source and/or actuator. In fact, the actuator needed is so small, that for most embodiments, it can be internalized within the mount. While the actuator enables the mount to become a more efficient isolator for one frequency or range frequencies, it also permits the same mount to be an effective damper for other frequencies. Indeed, for an application where the mount encounters only one or two primary excitation frequencies, the actuator can be controlled in such a manner as to drive the dynamic stiffness of the mount to zero over a wide range of frequencies, say, for example, from 20-200 Hz.
This device may be used as a tuned absorber for a variety of applications. In a typical active absorber case, force is input through the actuator to cancel essentially all of the vibrational motion at the point the absorber is attached to the structure. Typically, the energy of the absorber will be 180.degree. out of the phase with respect to the effect of the vibrational energy of the source at the point of attachment of the absorber. If the absorber is operated at a frequency near resonance, the available cancellation force will be maximized.
Various other features, advantages and characteristics of the present invention will become apparent after the reading of the following detailed description.